An image sensor is a semiconductor device used to convert optical images detected by the image sensor to electric signals. Image sensors may be classified as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS).
A CCD image sensor is provided with metal oxide silicon (MOS) capacitors that are spatially positioned within close proximity to each other and charge carriers are stored in and transferred to the capacitors.
A CMOS image sensor is provided with a plurality of MOS transistors corresponding to pixels of a semiconductor device having a control circuit and a signal processing circuit as peripheral circuits. The control circuit and the signal processing unit may be integrated together to employ a switching method that detects output through the MOS transistors.
CCD and CMOS image sensors may each be provided with a plurality of color filters such as a color filter array (CFA) on the upper surface of a photodiode to receive light and generate and store photocharges. A CFA may include three color filters such as a red color filter, a green color filter and a blue color filter. The CFA may alternatively include a yellow color filter, a magenta color filter, and a cyan color filter. Moreover, the image sensor may include a photodiode for sensing light and a logic circuit for processing the sensed light into an electric signal in order to create data.
In order to increase photosensitivity, a fill factor of the ratio in which the photodiode occupies the image sensor may be increased. However, increasing the fill factor could result in the lack of space of the logic circuit, and thus, increasing the fill factor ratio may be limited within a restricted area.
Accordingly, in order to increase the photosensitivity, a light condensing technique may be implemented for changing an optical path of light entering a region other than the photodiode for condensing the light to the photodiode. For such light condensing, the image sensor may include a microlens formed on and/or over the color filters. A photoresist may be formed on the uppermost metal pad that may serve as a bonding pad, however, a chemical reaction during processing may lead to surface contamination at the uppermost metal pad. This, in turn, may result in defective semiconductor devices having inferior bondability of the wire bonding.
As illustrated in example FIG. 1, an uppermost metal pad 100 contaminated by a plurality of contaminants 110. When final processing for the formation of metal wiring such as the deposition of metal and patterning/etching, a protective film may be formed for protecting the semiconductor device from external shock and from permeation of natrium ions Na+ and moisture. A dual layer protective film may be formed by depositing an oxide film 120 and a nitride film 130.
Etching may be carried out to expose the metal pad using a carbon-based gas or a fluoride-based gas. If a fluoride-based gas is used, any fluoride remaining after etching can react with aluminum. Accordingly, although a metal polymer can be removed by performing a solvent cleaning after the etching, the fluoride remaining on the surfaces is not thoroughly removed. In essence, as illustrated in example FIG. 2, an AlxFx type impurity A may be generated due to the reaction between aluminum and fluoride. Impurity “A” may in turn react with oxygen or other gases and transformed into another foreign substance. Such impurities can cause inferior bondability of the wire during bonding.